The present invention relates generally to the monitoring of output power of a semiconductor laser diode and, particularly, to the monitoring of the output power of a laser diode by detecting radiation emitted from the laser diode back facet.
It is well known in the scanning art to use laser diodes to generate a coherent laser beam which is optically shaped and used to scan a recording medium surface. It is also known to use multiple laser diodes to create multiple beams, each individual beam independently modulated by video signals, and the multiple beams scanned onto the recording surface as modulated beams. For these multiple beam applications, it has been found advantageous to use arrays of closely spaced laser diodes. Closely spaced diodes allow for multiple beam processing and thus improve data throughput as compared with systems that employ continuous wave, single beam gas or laser diodes.
Typically, the laser diodes in a multiple beam system are individually addressable. Individual addressability generally requires that each diode have a separate current source that drives or modulates the diode. In operation, each driver sends a current through the diode sufficient to induce emission of laser light. The amount of current the driver produces is determined, in part, by the digital input data driving that particular lasing element. An example of a Raster Output Scan (ROS) system using a dual laser diode is disclosed in U.S. Pat. No. 4,796,964, whose contents are hereby incorporated by reference.
Because different laser diodes have different output power characteristics in response to a given driving current, it is desirable to monitor the amount of output power from each laser diode. If it is found that a particular diode is outputting too much or too little power at a given current level then the current needs to be adjusted to correct for the power differential. Laser diodes are typically constructed layer by layer from epitaxial deposition of appropriately doped semiconductor material. The front and back facets are then cleaved to produce reflective surfaces that define the front and back boundaries of the laser cavity. The front facet is designed to be much more transmissive than the back facet, the back facet generally made to be highly reflective. The majority of laser light is emitted from the front facet.
As stated above, the back facet is designed to be a highly reflective surface. However, some light does ultimately escape through the back facet of the diode. The amount of light leakage through the back facet is generally known to be directly proportional to the amount of light emitted from the front facet. This relationship between radiation from the back facet and the radiation from the front facet affords the opportunity to monitor the amount of output power from the front facet by detecting light emitted from the back facet.
To measure the amount of light from the back facet of a laser diode, a detector is typically disposed opposite the back facet of a single laser diode. In the case of a single laser diode configuration, one back facet detector gives complete information concerning the amount of radiation emanating from the front facet of that diode. In a multi-diode configuration, the confluence of concurrent, multiple beams on a single back facet detector does not give information concerning any particular diode and various measures must be taken to extract appropriate signals.
FIG. 1 shows a side schematic view of a prior art ROS system utilizing back facet detection feedback for controlling the diode output. Input video data is transmitted to a self-modulating light source 12, such as a low powered solid state laser diode, which produces a modulated diverging beam of coherent light. The diode is driven in accordance with image signals entered into and processed by ESS 17. The beam is collimated by a spherical collimating lens 14 and is next incident upon a cylindrical lens 15 which focuses the light to a line image in the fast scan direction onto a rotating polygon 20 having at least one mirrored facet 21. The rotation of the mirrored facets causes the beam to be deflected and thereby scanned across a photosensitive image member which is shown as a photoreceptor drum 24. Postscan optics system 22 reconfigures the beam reflected by facet 21 to a circular or elliptical cross-section, refocuses the beam to the proper point on the surface of drum 24, and corrects for scan nonlinearity (f-theta correction). A 1.times. (or other working magnification) toroidal lens 28 or cylinder mirror (not shown) is disposed between the scanning device 20 and the photoreceptor 24 to correct for wobble (scanner motion or facet errors) where appropriate.
The laser diode 12 has front and back facets. While the majority of the laser light escapes from the front facet, some radiation is emitted from the back facet of the diode. This radiation is detected by a photodiode 32 which generates output signals which are sent into feedback circuit 44. This signal is compared to a predetermined voltage level corresponding to the desired power output of the laser diode. If correction is needed, a signal is sent to the laser diode drive circuit to increase or reduce the emitter power output.
The problem with this prior art laser diode output monitoring system is that, for optimum light output control, real time, pixel by pixel control of a laser diode is needed to eliminate exposure variations along the scan line due to thermal effects in the laser diode such as droop, creep, and changes in the bias level and the slope of the LI curve. This type of light output control is difficult to implement because of the extremely short pixel times involved (typically, on the order of tens of nanoseconds.) In order to make the control system fast, the response time of the detection and feedback system must be minimized. In the present system of FIG. 1, there are two factors which can slow the response time of the detection system; one factor is driven by the electronics and the other is dependent on optical considerations.
The electronic factor involves the back facet photodiode circuit design. There are two design considerations which determine the response time of a photo diode circuit. First, the photodiode must be reverse-biased with the amount of biasing being proportional to a decrease in response time. The other design consideration for response time is the capacitance of the photo diode. The higher the capacitance, the higher the response time (low bandwidth). To design a fast photodiode circuit, it is necessary to limit the effect of the photodiode capacitance on the circuit. In a typical dual laser diode configuration, the cathode of the back facet photodiode is connected to ground, as well as to the laser diode case (for heat sinking), and the laser diode is driven with a positive current. The common connection to ground in this design configuration makes it impossible to reverse bias the back facet photodiode with a negative voltage at its anode and measure the current directly out of its cathode. Since both the anode and the cathode are connected to a voltage, it is impossible to directly measure the current out of the back facet photo diode. The only way to measure the photo diode current (monitor)is to indirectly measure the voltage across a dropping resistor (r1). The response time of this circuit is limited to the photo diode capacitance times the resistor (r1) value (RC time constant). Since the photodiode puts out a relatively small current the value of r1 needs to be relatively large to produce a usable signal. The RC time constant of this circuit severely limits the bandwidth of this circuit.
The optical factor which slows the response time involves the size of the back facet photodiode 32, which is proportional to its capacitance. The larger the photo diode the larger the capacitance. The back facet detector in a typical laser diode package configuration is inherently slow because it is relatively large, on the order of 0.64 mm.sup.2. The size of one commercial detector, a Motorola MRD500 which has a fast response is 0.025 mm.sup.2. A larger back facet detector is used, in the conventional laser diode package configuration, because it is positioned approximately 2 mm from the back facet of the laser diode, and must therefore be large to collect sufficient light from the back facet emission to produce an adequate signal to noise ratio at the output from the back facet detector.
The above problems are compounded in a laser diode system having two or more laser emitter outputs. The output from one of the laser diodes must be monitored while the other laser diode is being modulated. Thus, the light output by a first diode must be detected independent of the light output by the second diode. In currently available dual laser diode packages, a single photodiode is used to monitor both laser diodes. This configuration makes real time control impossible. It is apparent that it would be more efficient to be able to separately and simultaneously measure the light intensity of each laser diode emitter.
Thus, there is a need to construct an array architecture such that the amount of light emitted from individual back facets, of a multiple laser diode configuration is detected by corresponding photodiodes. Additionally, there is a need to regulate the output of the individual diodes in a continuous closed loop configuration to insure high print quality.
It is thus a first object of the present invention to provide a back facet monitoring system such that the amount of output power from individual back facets of laser diodes can be individually monitored in a continuous fashion with a relatively fast response time.
There are additional prior art problems in back facet monitoring. In typical laser designs, the back facet is coated with a reflective material with 99.5% reflectivity. Thus only a small amount of light (0.5%) is emitted from the back facet and is available to measure the power. Thus, photodiode 32 in FIG. 1, which is typically placed several millimeters behind laser 12, collects only a relatively small fraction of the already reduced light emitted from the laser 12 back facet.
It is therefore a second object of the present invention to increase the sensitivity of the back facet light detector.
Another problem of prior art systems is stray light impinging on the photodiode 32 distorting the output signal. The stray light is the result of reflections from the optical components in the system (e.g., from the optical components in lens 24) being reflected from the rear facet and onto the detector. The detected signal will be distorted due to the optically induced "noise". For multiple diodes, increased "cross talk" results.
It is therefore a still further object of the invention to reduce the effects of stray light interference on back facet power monitoring detector signals. These and other objects are realized by replacing the single prior art large sized back facet photodiode with two smaller photodiodes which are mounted relatively close to the back facets of the laser diodes being monitored. In one embodiment, each photodiode has orthogonally oriented polarizers formed over the detecting face to shield each photodiode from the back facet light emitted by the laser diode it does not monitor. An improved optical feedback circuit is used to control each laser diode output power and bias level in response to the signals detected by the respective photodiode. In another embodiment, in addition to the orthogonally oriented polarizers over each photodiode a polarization rotating half wave plate is positioned between one of the diodes and its corresponding photodiode enabling a simplified control circuit and relaxing the specification and crosstalk between the two laser diodes. More particularly, the present invention relates to an apparatus for monitoring the power output of at least a first and second laser diode each having at least a front and back facet, said laser diode generating output beams having different polarization characteristics, said apparatus including:
at least a first and second photodiode associated with said first and second laser diodes, said photodiode positioned so as to receive at least a portion of the light emitted from the back facets of said laser diodes;
means for selectively changing the polarization of at least one of said output beams whereby the light output from back facets are selectively sensed by said photodiodes; and
means for monitoring the output of said photodiodes and for adjusting the power outputs of said laser diodes to maintain the desired level.